Spatial dependencies of nuclear magnetic resonance (NMR) phenomena on the macroscopic scale are the basis for medical diagnostic imaging, and separately measured departures from homogeneity are important for analytic purposes. The acquisition of spatial distributions of nuclear magnetic resonance properties includes methods based upon a reconstruction methodology, e.g. tomography, and also methods based upon a phase encoding principal as exposited in U.S. Pat. No. 4,070,611, commonly assigned. Both classes of NMR experiments involve rapidly switched magnetic gradients to effectuate a mapping of spatial coordinates to resonant frequency for the acquisition of some magnetic resonance parameter as a function of spatial coordinates.
A technique for magnetic resonance mapping, for which rapidly switched magnetic field gradients are not essential, is known from the work of Hoult, J. Mag. Res., Vol. 33, pp. 183-197 (1979); Cox and Styles, J. Mag. Res., Vol. 40, pp. 209-212 (1980). This approach is called rotating frame zeugmatography (RFZ). There are certain technical advantages in the RFZ approach, notably in dispensing with requirements for rapidly attaining precision magnetic field gradients and, as well, for avoiding operation of eddy currents in the object under study and the surrounds. It should be understood that static and/or switched field gradients may be used to advantage in RFZ measurements.
The operative spatial localizing principle for an experiment of the RFZ type is provided by utilizing an RF field having a selected gradient. If desired, the static polarizing field may remain homogeneous or may include magnetic field gradients depending upon the type of measurement. The RF gradient field has been produced by asymmetric saddle coils, surface coils and the like. The nutation angle of the resonating spins therefore varies over the spatial coordinates of the sample in accord with the RF field distribution. The nutation angle is a bi-linear function of both B.sub.1 (the local RF magnetic field) and the pulse length of the RF irradiation; consequently, each volume element of the sample varies systematically with the pulse length, and/or pulse amplitude. A Fourier transformation in pulse length (or pulse amplitude) yields a spatial distribution in one dimension for each chemical shift value.
For off-resonant conditions, it is common to observe artifacts of three common varieties in an RFZ measurement of prior art. First, there is typically observed an asymmetry occurring in chemical shift spectra between positive and negative frequency components of the same spectral object; second, off-resonance situations give rise to a spectral contribution at zero frequency; and third, the peak position is found to shift with displacement for resonance.
In the basic experiment dealing with spatial distribution of NMR phenomena, one obtains a digitized waveform for each of a controlled number of excitations, n. A transient wave form describing the de-excitation of the resonating nuclei is observed for each such excitation. A set of such waveforms are obtained by varying some other parameter to provide a second independent variable. In the present invention that second independent variable may be either the RF power or RF pulse duration.